We propose to continue work along the general lines of previous research (GM 11223-01 to 10), applying fluorescence techniques to proteins and other systems of biological significance. We shall proceed in three main directions: 1. By undertaking further study of relevant fluorophores with improved accuracy in the measurements of spectral distribution, fluorescent yield, lifetime and polarization. We shall study the rotational diffusion of small irregular fluorophores and of proteins using real-time depolarization, and will continue studies of fluorescence quenching by oxygen. 2. By development of new methods of observation of particular biological interest: We plan the construction of a variable frequency cross-correlation fluorometer which will permit determination not only of the average lifetime but of the dispersion of lifetimes values in natural systems. We plan also to construct a fluorescence spectrometer for observation of solutions at a pressure of several kilobars. 3. We shall also explore the new areas of ligand- protein interaction predicted by recent theory, and of which we have obtained convincing experimental evidence.